In recent years, drawing attention as reasonable data services for corporate use is, wide area Ethernet (registered trademark) VPN service (wide area Ether) which is an expansion of Ethernet (registered trademark) techniques widely used in related art LAN to a wide area network. Wide area Ether takes over such advantages of the related art Ethernet (registered trademark) techniques including easy to use as Plug and Play and low costs.
FIG. 49 shows one example of a wide area Ether network. The wide area Ether network is formed of edge switches E1, E2, E3 and E4 which accommodate user terminals T1, T2, T3 and T4, respectively, and core switches C1 and C2 which execute relay operation only without accommodating a user terminal. Proposed as a system for transferring a frame in a wide area Ether network other than an ordinary Ethernet (registered trademark) frame transfer system is a method recited, for example, in Literature 1 to be described later in which with node ID assigned to the edge switches E1 through E4, each node in a wide area Ethernet (registered trademark) network transfers a frame based the node ID. In the method recited in Literature 1, with respect to a frame received from the user terminal T1˜T4, an Ingress edge switch stores a node ID of an Egress edge switch to which the destination user terminal T1˜T4 is connected in a VLAN tag field (hereinafter referred to as an expansion tag), each node in the wide area Ethernet (registered trademark) network transfers a frame based on the expansion tag and the Egress edge switch transfers the frame to the user terminals T1 through T4 with the expansion tag deleted from the frame. In the example shown in FIG. 49, g1, g2, g3 and g4 are set in the edge switches E1, E2, E3 and E4 as their node ID.
With reference to FIG. 50 and FIG. 51, frame format will be described. FIG. 50 shows a format of an Ethernet (registered trademark) frame 200. The Ethernet (registered trademark) frame 200 is formed of a destination MAC address 210, a transmission source MAC address 220, a VLAN tag 230, Type 240, a payload 250 and FCS 260. On the other hand, FIG. 51 shows a format of an expansion tag frame with an expansion tag added. An expansion tag frame 300 has an expansion tag 310 inserted between the transmission source MAC address 220 and the VLAN tag 230 in the Ethernet (registered trademark) frame 200. In FIG. 50 and FIG. 51, there is a case where the VLAN tag 230 is not added. In the present specification, description will be made on the premise that the VLAN tag 230 is added.
According to the Ethernet (registered trademark) techniques, without any measures, when a loop structure exists in a network, a frame might continue circulating on the loop, so that the network might go down particularly when a broadcast frame continues circulating. For avoiding this situation, even when a loop structure exists in the network, a spanning tree protocol (hereinafter referred to as STP, which technique is defined by IEEE802.1D) for forming a loop-free network or a rapid spanning tree protocol as a high-speed operation version of the same (hereinafter referred to as RSTP which technique is defined by IEEE802.1w) are used in may cases with a loop logically excluded. When using STP or RSTP, any of ports in a loop structure enters a blocking state (state where neither transmission nor reception of a main signal frame is executed, more precisely, in which while a frame is transferred, the frame is abandoned at a port in the blocking state), thereby making the structure loop-free. In the network shown in FIG. 49 as an example, while a loop structure exists among the edge switch E3, the core switch C1, the core switch C2 and the edge switch E4, attaining the blocking state by a port p2 of the core switch C2 makes the structure loop-free. In a case where such STP or RSTP is used, however, because a link connected to a blocking port is not allowed to transfer a frame, when transferring a frame between certain switches, the frame can not be transferred by the shortest path (path with a minimum number of hops). In the example shown in FIG. 49, when transferring a frame from the user terminal T2 to the user terminal T1, because a port p1 of the edge switch E2 is in the blocking state, a frame, which can not be transferred by a path from the edge switch E2 to the edge switch E1, will be transferred by a path from the edge switch E2, the core switch C2, the edge switch E4, the edge switch E3, the core switch C1 and the edge switch E1 to arrive at the user terminal T1. In other words, although the distant is one hop in terms of physical topology, five hops should be passed in terms of logical topology to prevent shortest path transfer in some cases.
Recited as a technique for solving the problem in Literature 2 is a method in which with a Multiple STP (hereinafter referred to as MSTP) capable of managing a plurality of STP/RSTP for each VLAN used, each edge switch generates STP/RSTP with its own switch as a route node, thereby making a transfer path of a frame whose destination is an edge switch which will be a route node of each STP/RSTP be its STP/RSTP. Since a link brought to be active in STP/RSTP (link not including a blocking port) is selected to be one whose link cost from a route node is the minimum, use of the method recited in Literature 2 enables transfer by a shortest path. Shown in FIG. 52 is an example of use of the method recited in Literature 2 for the above frame transfer from the user terminal T2 to T1 described with reference to FIG. 49. In FIG. 52, frame transfer from the user terminal T2 to T1 is executed by using STP/RSTP with the edge switch E1 as a route node (transfer from the edge switches E3 and E4 to E1 is also executed by using STP/RSTP with the edge switch E1 as a route node). Accordingly, a frame from the user terminal T2 arrives at the user terminal T1 via the edge switch E2 and the edge switch E1. Thus, transfer between the respective nodes can be realized by a shortest path.
For realizing such transfer as described above, recited in Literature 2 is such processing as follows. In transfer between edge switches, with a node ID set in each edge switch stored in a VLAN tag, each edge switch and core switch transfer a frame based on the ID. In FIG. 52, with the node ID g1, g2, g3 and g4 assigned to the edge switches E1, E2, E3 and E4, respectively, as described above, in transfer from the edge switch E2 to the edge switch E1, the VLAN tag g1 is stacked in a frame (this VLAN tag will be denoted as an expansion tag) at the edge switch E2, so that the edge switch E2 transfers the frame toward the edge switch E1 based on the expansion tag g1. In a forwarding table of each switch, an output port for an expansion tag value is managed, in which set as an output port is a port number of a route port (state of the port then is a forwarding state indicative of a transfer allowed state) of STP/RSTP whose route node is an edge switch having a node ID equivalent to an expansion tag value. In FIG. 52, for an output port for the expansion tag g1, each switch sets a port number of a route port in the forwarding state in STP/RSTP whose STP-ID is g1. Similarly, for transferring a frame whose destination is the user terminal T2, STP/RSTP with the edge switch E2 as a route node will be a transfer path, for transferring a frame whose destination is the user terminal T3, an STP/RSTP tree with the edge switch E3 as a route node will be a transfer path and for transferring a frame whose destination is the user terminal T4, STP/RSTP with the edge switch E4 as a route node will be a transfer path. Configurations of the respective transfer paths are illustrated in FIG. 53 (A)˜(D). Thus, according to a frame destination user terminal, making an STP/RSTP tree whose route node is an edge switch to which the user terminal is connected be a transfer path enables a frame transfer path for any node to be an optimum path.
Literature 1: Hidaka et al., “Proposal of Next Generation Ethernet (registered trademark) Architecture GOE (Global Optical Ethernet (registered trademark))—(1) Basic Concept•Framework•Element Technique”, Institute of Electronics, Information and Communication Engineers of Japan, Society Conference 2002, B-7-11.
Literature 2: Umayabashi et al., “Proposal of Next Generation Ethernet (registered trademark) Architecture GOE (Global Optical Ethernet (registered trademark))—(2) High Efficiency Routing and High-speed Protection”, Institute of Electronic, Information and Communication Engineers of Japan, Society Conference 2002, B-7-12.
Noting a certain transfer path, however, finds the following shortcomings.
In a case, for example, of a path whose destination node is the edge switch E1 shown in FIG. 53(A) (in a case of a transfer path of a frame whose destination is a user terminal connected to the edge switch E1), although selected links form the shortest path to the edge switch E1, due to original properties of STP/RSTP, setting of a port in the blocking state causes a link which can not be used for transfer to exist.
In the example shown in FIG. 53(A), such link corresponds to a link between the core switch C1 and the core switch C2 and a link between the edge switch E3 and the edge switch E4. Also in FIG. 53 (B)˜(D), there exist links which can not be used for frame transfer. In the example shown in FIG. 53, in particular, the link between the core switch C1 and the core switch C2 is used in none of transfer paths in frame transfer (whether there exists such a link not used at all as described above or not depends on parameters including topology, link costs and a port number).
In other words, path setting recited in Literature 2 enables shortest path transfer, while there is a room for improvement in link use efficiency.
An exemplary object of the present invention is to provide a node, a network system, a frame transfer method and a frame transfer program which enable an improvement in throughput of a network as a whole while executing shortest path transfer.